Modern manufacturing processes for cylindrical products enable high production throughrates to be maintained for substantial time intervals, in pressing, forging, stamping or molding such items. As the production system is operated, wear and distortion are introduced until a condition is reached at which the formed parts are no longer within established tolerances. Very often, the parts must have precisely controlled wall dimensions and curvatures to avoid catastrophic or damaging results in use. Open ended cylindrical members such as large heavy shell casings and other cylinders must have substantially uniform walls throughout their perimeter, to prevent overheating and jamming. At another extreme, aluminum cans used in the beverage industry must also be precisely controlled, because excess wall thickness is wasteful and costly, and inadequate wall thickness can involve sudden rupture.
The production of aluminum beverage cans using multiple high speed presses provides a good example of modern wall thickness measurement problems. Usage rates today for these cans are such that the typical facility comprises a number of high speed presses in parallel, each operating at high cycle rates (up to 1200 cans per minute) to form open ended cylinders which comprise all of the can body apart from the top, which is attached separately and later. In production, the can walls gradually change in thickness as the dies wear, until a point is reached at which that press must be shut down and the dies replaced or adjustments made. Although long term trends can sometimes be monitored, die life and press performance can vary substantially, and at times product changes can take place rapidly. The present approach toward quality assurance is for operators to manually select samples and then gauge wall thickness using micrometers. No matter how skilled and mechanically adept the operator, however, it is not feasible for him to keep current with the output rates of a number of high speed presses.
Furthermore, it is preferable to acquire more accurate and reliable data as to a number of can characteristics, using more sensitive measurement devices and eliminating operator judgments and mistakes. It is desirable to measure various critical curvatures at the top and bottom of the can, to gauge significant dimensions, and to take many wall readings, in effect mapping the wall thickness precisely at several hundred or more positions around and along the wall. Because the walls not only vary in thickness but are wavy along their length, and because the precision needed for the measurement of the order of one micron (40 microinches) or less, the measurement problems are greatly compounded. Under these conditions, temperature effects, wall undulations, and other variations can introduce unacceptable errors in measurements. The wall is so thin that contact with a harder member can introduce scratches which themselves introduce errors. As cans emanate at high production rates from a group of presses, a non-contact measurement system is needed that selectively samples and transfers the cans, identifies the cans in accordance with the originating press, and maps wall thickness rapidly at a substantial number of points about the can, with sufficient accuracy to generate meaningful data as to press performance in real time.